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Chapter 40 More About....
IP Addresses and Subnet Masks
40-2
Cisco Router and Security Device Manager 2.5 User’s Guide
OL-4015-12
The subnet mask is used to specify how many of the 32 bits are used for the
network number and, if subnetting is used, the subnet number. It is a binary mask
with a 1 bit in every position used by the network and subnet numbers. Like the
IP address, it is a 32-bit value, expressed in decimal format. The following figure
shows a subnet mask entered in Cisco SDM. Cisco SDM shows the subnet mask
and the equivalent number of bits in the mask.
These values entered Cisco SDM represent the binary mask shown in the
following figure:
This subnet mask specifies that the first 24 bits of the IP address represent the
network number and subnet mask, and that the last 8 bits represent the host
number within that network and subnet. You can enter the mask in the dotted
decimal format shown in the Subnet Mask field, or you can select the number of
bits in the bits field. When you enter or select a value in one field, Cisco SDM
automatically adjusts the other.
Cisco SDM displays a warning window if you enter a decimal mask that results
in binary zeros (0s) in the network/subnet area of the mask. The following subnet
mask field contains a decimal value that would result in binary zeros in the
network/subnet number portion of the mask. Note that the bits field on the right
is empty, indicating that an invalid value has been entered in the Subnet Mask
field.